In the processing of semiconductor substrates, such as silicon wafers, to make devices, many steps of heating and cooling the wafer are used. During these processing steps, the heating and cooling operations produce a non-uniform temperature distribution in the wafer which results in thermoelastic stresses. When such thermoelastic stresses are forced to go beyond the strongly temperature-dependent yield stress, a phenomenon known as plastic flow occurs along the slip planes of the crystalline structure of the wafer. The stress causing this plastic flow results not only in a high dislocation density of the structure of the wafer, but also a permanent strain in the form of an elongation within regions close to the rim or peripheral portions of the wafer causing warpage of the wafer. These problems become more severe as the size of the wafer increases.
Heretofore, the techniques for detecting warping depended upon the use of geometric measurements, such as profilometers. Moreover, in order to determine the location of slip planes, etching techniques are needed to render them visible at the surface. However, this is a destructive test. Other techniques, such as x-ray topography and special electron microscopy techniques for observing dislocations are too sophisticated and take too much time for wafer testing as a routine process. Therefore, it is desirable to have a technique for determining the location and extent of slip planes, which technique is relatively easy to carry out and which is non-destructive of the wafer.